struggling about the Chest, Chin and elbow on the line and the shoulder behind head

[masterqq]

http://www.youtube.com/watch?v=-PaxvTFlez8
http://www.youtube.com/watch?v=1_DZD3Ie__o
http://www.youtube.com/watch?v=0yhGJNaRP6s

Here are the video film by my friend.
I found that my shoulder and elbow cannot hide behind the head on the straight line with my grip hand.

i really confusing about this because i feel that it affects my cue action on the straight line.

Please help >